Twister designed magnetic field generators have been provided by current carrying coils of very high amperage adapted to produce helically varying transverse magnetic fields of the magnetization desired. In recent developments, notably U.S. Pat. No. 4,764,773, incorporated herein by reference, permanent magnets have been designed and arranged in certain specific ways to form structures which produce desirable helical or "twisted" fields obviating the need for commonly used current carrying coils with their attendant weight and space problems. These structures are based upon the hollow cylindrical flux source (HCFS) principle described by K. Halbach in "Proceedings of the Eighth International workshop on Rare Earth Cobolt Permanent Magnets", Univ. of Dayton, Dayton, Ohio, 1985 (pp. 123-136). A HCFS, also referred to sometimes as a "magic ring", is essentially a cylindrical permanent magnet shell that produces an internal magnetic field which is relatively constant in magnitude. The field, which is perpendicular to the axis of the cylinder, (transverse) possesses a strength which can be greater than the remanence of the magnetic material from which the ring is made.
Ideally, the HCFS is an infinitely long annular cylindrical shell with a circular cross section, which produces an intense magnetic field in its interior working space. No magnetic flux extends to the exterior of the ring structure (except at ends of a finite cylinder). A HCFS is not limited to the ideal cylindrical structure, but may be represented by octagonal, sixteen sided, thirty-two-sided, and even higher order polygonal-sided structures which approximate the ideal HCFS structure.
In "twister" structures there also exists an undesirable longitudinal component of the magnetic field in combination with the transverse component, arising from the high helical motion, i.e. "frequency". As the frequency increases, the longitudinal component increases, weakening the transverse component. Therefore, it has been of increasing concern to produce stronger transverse magnetic fields in "twister" configurations.